The present invention relates to optical amplifiers having operating wavelengths longer than main emission peak wavelengths, and more particularly to erbium doped fiber and waveguide amplifiers operating in the long wavelength regime (1560-1620 nm), especially for wavelength division multiplexing (WDM) applications.
Conventional erbium doped fiber amplifiers (EDFA) have been extensively used in optical telecommunications as means to amplify weak optical signals in the third telecommunication window (near 1550 nm) between telecommunication links. Much work has been done on the design of these amplifiers to provide efficient performance, such as high optical gain and low noise figure. However, with the recent enormous growth of data traffic in telecommunications, owing to the Internet, intranets, and e-commerce, new optical transmission bandwidths are required to provide increased transmission capacity in dense wavelength division multiplexing (DWDM) systems.
There are a few solutions to this demand. One proposed solution is to utilize new materials compositions as a host for the fiber gain medium (instead of silica) such as telluride, which may provide broader amplification bandwidth (up to 80 nm). However, the non-uniform gain shape and poor mechanical properties of telluride glass make these amplifiers difficult to implement in the telecom systems. Also, Raman amplifiers can be considered as an alternative solution to high bandwidth demand, since these amplifiers are capable of providing flexible amplification wavelength with a broad bandwidth. However, these amplifiers place restrictions on optical system architectures because of their required designs for efficient performance, such as long fiber length ( greater than 5 km), high pump power ( greater than 500 mW) and co-pumping configurations. On the other hand, relatively long erbium doped fibers (EDFs) may also provide amplification in the long wavelength range (1565-1625 nm) when they are used with high power pump sources. This range is commonly called xe2x80x9cL bandxe2x80x9d. The conventional range, also known as xe2x80x9cC bandxe2x80x9d is in the wavelength range between 1525-1565 nm.
In principle, L band amplifiers take advantage of the fact that EDFs have a higher emission cross-section than absorption cross-section at longer wavelengths. Therefore, for long EDFs, amplified spontaneous emission (ASE) becomes more emphasized at long wavelengths. However, there are still several issues for optimization of L band amplifiers for efficient performance. So far, reported performance of L band EDFAs has been inferior to that of C band EDFAs, with drawbacks as evidenced by higher noise figure (NF) and lower output power and gain.
European Patent Application EP 0954070 A2 to Terahara et al. discloses an optical amplifier that uses both co-pumping and counter-pumping of a gain medium, with 1550 nm ASE reflectors on either side of the gain medium, which reflect the ASE being transmitted from the gain medium back into the gain medium. However, such an arrangement promotes a lasing effect, which produces a gain clamped amplifier, limiting the achievable gain of the amplifier.
It would be beneficial to provide an L band amplifier with a low noise figure and high output power and gain.
Briefly, the present invention provides an L band optical amplifier. The amplifier includes a signal line for transmitting a signal light in a first direction. The signal line includes an input, an output disposed optically downstream of the input, and a first amplifying gain medium optically disposed between the input and the output. A first laser is optically aligned with the first amplifying gain medium to transmit a first pump light to the first amplifying gain medium toward the output. A first reflector is disposed along the signal line between the first amplifying gain medium and the output to reflect a first bandwidth of light from the first amplifying gain medium back into the first amplifying gain medium. A second laser is optically aligned with the first amplifying gain medium to transmit a second pump light to the first amplifying gain medium toward the input. A second is reflector disposed along the signal line between the first amplifying gain medium and the input to reflect a second bandwidth of light, different from the first bandwidth of light, from the first amplifying gain medium back into the first amplifying gain medium.
Additionally, the present invention provides a method of amplifying a light signal. The method comprises amplifying an L band optical signal comprising transmitting an L band optical signal through an amplifying gain medium in a first direction; transmitting a first pump signal into the amplifying gain medium in the first direction; transmitting a second pump signal into the amplifying gain medium in a second direction, opposite the first direction, wherein the first and second pump signals amplify the L band optical signal by a first amount and generate amplified spontaneous emission in the amplified gain medium, wherein the amplified spontaneous emission is transmitted from the amplifying gain medium in each of the first and second directions; reflecting a first bandwidth of the amplified spontaneous emission from the first direction to the second direction and back into the amplifying gain medium; and reflecting a second bandwidth of the amplified spontaneous emission, different from the first bandwidth, from the second direction to the first direction and back into the amplifying gain medium, wherein the reflected first and second bandwidths further amplify the L band signal by a second amount in the amplifying gain medium.
Further, the present invention provides an L band optical amplifier comprising a signal line for transmitting a signal light in a first direction. The signal line includes an input, an output disposed optically downstream of the input, and a first amplifying gain medium optically disposed between the input and the output. A first laser is optically aligned with the first amplifying gain medium to transmit a first pump light to the first amplifying gain medium toward the output. A first reflector is disposed along the first amplifying gain medium to reflect a first bandwidth of light from the first amplifying gain medium back into the first amplifying gain medium. A second laser is optically aligned with the first amplifying gain medium to transmit a second pump light to the first amplifying gain medium toward the input. A second reflector is disposed along the first amplifying gain medium to reflect a second bandwidth of light, different from the first bandwidth of light, from the first amplifying gain medium back into the first amplifying gain medium.
Also, the present invention provides an L band optical amplifier comprising a signal line for transmitting a signal light in a first direction. The signal line includes an input, an output disposed optically downstream of the input, and an amplifying gain medium optically disposed between the input and the output. A laser is optically aligned with the first amplifying gain medium to transmit a pump light to the amplifying gain medium toward the input. A reflector is disposed between the first amplifying gain medium and the output to reflect a bandwidth of light from the amplifying gain medium back into the amplifying gain medium.